The present invention relates generally to a system for detecting the initial point of a digital signal in a composite digital signal reproducing system, and more particularly to a system for detecting the initial point of a digital signal which is formed from an information signal such as an audio signal and exists during a period corresponding to the video signal period between adjacent composite synchronizing signals of a composite video signal.
Known recording and reproducing apparatuses for audio signals which record and reproduce analog audio signals, as they are, on and from a traveling magnetic tape by means of a fixed head, have experienced such problems as wow and flutter of the tape travel, noise, distortion, etc., due to the tape and head systems. As a consequence, these problems have constituted a barrier limiting efforts to improve the quality of recording and reproducing of audio signals.
Accordingly, a method wherein an analog audio signal is rendered into a digital signal by using a method such as pulse code modulation (PCM) has been developed and put into practice. By this method, the S/N ratio is greatly improved, and the effects of problems such as distortion due to nonlinearity of the recording medium are greatly reduced.
This recording and reproducing of a digital signal requires a recording and reproducing system of wider band or of a greater number of channels than a recording and reproducing system for an analog method of carrying out recording and reproducing of a digital signal using a video tape recorder (VTR) of the type widely used for recording and/or reproducing composite audio signals, and easily available on the market, has been developed and reduced to practice. In this specification, the term "composite video signal" is used to mean a signal composed of an information video signal and synchronizing signals such as vertical synchronizing signals, equalizing pulses, and horizontal synchronizing signals. A VTR of this type is capable of recording/reproducing video signals in a wide band with a track oblique to the traveling magnetic tape by means of rotary heads which scan with a relatively high scanning speed.
To record an audio signal with a VTR, the analog audio signal to be recorded is converted into a digital signal in an adapter device connected to the VTR, and this digital signal is interposed between synchronizing signals of the same kind as those of an ordinary composite video signal. The composite digital signal thus obtained is fed to the VTR and recorded on the magnetic tape by the rotary heads. At the time of reproducing, the signal reproduced from the magnetic tape by the rotary heads in the VTR is fed to the adapter device, where the synchronizing signals are removed therefrom, and the digital signal is converted into an analog signal and thus restored to the original audio signal.
In general, upon interposing the above-described digital signal between the synchronizing signals, in order to assure that the reproduced demodulated signal does not suffer any deficiency even when signal dropout has occured, the words obtained by each sample are arranged into a combination of words wherein each of serial words is apart from each other by ten-several H (H denotes the unit horizontal synchronizing period) to interleave with each other. Here, a "unit word" refers to the combination of bits obtained from a single sample. The period of time represented by ten-several H is determined in conformance with the format. In the reproducing system, writing-in and reading-out operation in and from a memory are carried out to rearrange the words which have been interleaved and changed in order are restored to the original order, whereby reproduction is accomplished.
VTRs for recording and reproducing digital signals are adapted to record and reproduce video signals composed of odd fields and even fields. Accordingly, a field-unit signal in which the digital signal is interposed between the composite synchronizing signals is similarly composed of an odd field and an even field, which differ by 0.5 H from each other.
In this connection, it is required to accurately detect the initial point of the digital signal for each field at the time of rearranging the words in the reproduction system.
Until the present time, in order to accurately detect the initial point of the digital signal, an identification signal for indicating the initial point of the digital signal has been interposed immediately after the vertical synchronizing signal. This identification signal is detected in the reproduction system, and in this way the beginning of the digital signal is known.
This known system, however, entails a difficulty in that employment of an identification signal decreases the time available for the digital signal by the period of the identification signal and this degrades the signal transmission efficiency. Furthermore, the use of an identification signal is troublesome in that it must be a special and easily distinguishable signal so as to prevent erroneous detection. Furthermore, the identification signal has to be standardized among manufactures so as to assure interchangeability.